23

1. 23 The Digestive System: Part A

2. Gastrointestinal Tract Activities • There are six essential activities: • Ingestion – taking food into the digestive tract • Propulsion – swallowing and peristalsis • Peristalsis – waves of contraction and relaxation of muscles in the organ walls • Mechanical digestion – chewing, mixing, and churning food • Chemical digestion – catabolic breakdown of food • Absorption – movement of nutrients from the GI tract to the blood or lymph • Defecation – elimination of indigestible solid wastes

3. Histology of the Alimentary Canal • From esophagus to the anal canal the walls of the GI tract have the same four tunics • From the lumen outward they are the • mucosa • submucosa • muscularis externa • serosa • Each tunic has a predominant tissue type and a specific digestive function

4. Enteric Nervous System • The GI tract has its own nerve supply – the enteric neurons • The system is made of interconnected ganglia found in the walls of the GI tract – short reflexes • Composed of two major intrinsic nerve plexuses: • Submucosal nerve plexus – regulates glands and smooth muscle in the mucosa • Myenteric nerve plexus – Major nerve supply that controls GI tract mobility • Linked to the CNS via afferent visceral fibers (long autonomic reflex arc) • The motor fibers of the ANS synapse with the enteric neurons

5. http://arbl.cvmbs.colostate.edu/hbooks/pathphys/digestion/basics/peristalsis.htmlhttp://arbl.cvmbs.colostate.edu/hbooks/pathphys/digestion/basics/peristalsis.html

6. GI Tract function regulation • Local mechanisms - Mechanical and chemical stimuli • Extrinsic control by CNS centers • Long reflexes – act through ANS • Intrinsic control by local centers • Short reflexes – stretching or chemical stimulation activates myenteric plexuses to stimulate contraction in the nearby regions • Hormonal mechanisms • Enhance or inhibit smooth muscle contraction

7. Receptors of the GI Tract • Mechano- and chemoreceptors respond to: • Stretch, osmolarity, and pH • Presence of substrate and end products of digestion • They initiate reflexes that: • Activate or inhibit digestive glands • Mix lumen contents and move them along

8. GI tract motility • Motility of the GI tract serves two purposes • Moving food from mouth to anus • Mechanical mixing of food to break it into small particles

9. Digestive system motility • The smooth muscle cells make groups connected electrically by gap junction • Form contracting segments • Visceral smooth muscle shows rhythmic cycles of activity • Pacesetter cells / interstitial cells of Cajal (ICC) • undergo spontaneous depolarization (slow wave potentials – 3-12 waves/minute as compare to 60-90 in the myocardial cells) • Wave frequency is different among the different areas – 3 in the stomach and 12 waves/min in the duodenum

10. Digestive system motility – types of contractions • Migrating motor complex between meals – tract is empty, wave begin in the stomach, each about 90 minutes to reach large intestine – “housekeeping” • Peristalsis – during and following meals – movement of 2-25 cm/second • Waves that move a bolus along the length of the digestive tract. • The circular muscle behind the bolus contract and the one ahead relaxes • Longitudinal muscles ahead contract next and shorten adjacent segments.

11. Digestive system motility – types of contractions • Segmentation – in the small intestine and some parts of large intestine. • A movement that mixes the bolus with intestinal secretion • That is a movement that does not push the materials in any direction

12. The esophagus • Carries solids and liquids from the pharynx to the stomach • Passes through esophageal hiatus in diaphragm • Travels through the mediastinum and pierces the diaphragm • Joins the stomach at the cardiac orifice

13. Histology of the esophagus • Esophageal mucosa contains stratified squamous epithelium • Changes to simple columnar at the stomach • Esophageal glands in submucosa secrete mucus to aid in bolus movement • Muscularis: skeletal superiorly; smooth inferiorly • Adventitia instead of serosa

14. Deglutition • Involves the tongue, soft palate, pharynx, esophagus, and 22 muscle groups • Buccal phase • Voluntary contraction of the tongue • Pharyngeal-esophageal phase • Involuntary • Control center in the medulla and lower pons

15. Cells type in the gastric glands • Gastric glands (fundus and body) • parietal cells • found mainly in the proximal portion of the gland • Secrete hydrochloric acid, intrinsic factor (glycoprotein that facilitate the absorption of vitamin B12 in the small intestine) • chief cells – pepsinogen, gastric lipase • Pyloric glands • mucus-secreting cells - mucus • Endocrine cells - secrete gastrin, histamine, endorphins, serotonin, cholecystokinin (CCK), and somatostatin into the lamina propria

16. Parietal cells - hydrochloric acid • the cells do not produce HCl (can destroy the cells). Hydrogen ion are the product of the function of carbonic anhydrase that convert CO2 into carbonic acid which dissociate into hydrogen and bicarbonate ions. • The bicarbonate ion is ejected to the bloodstream through a countertransport mechanism that exchange it with Cl ion (chloride shift) • The Cl ions diffuse through chloride channels into the gland lumen • Hydrogen ions are actively transported into the lumen of the gastric gland • HCl secretion does not affect the pH within the parietal cells because H+ is actively pumped out of the cell as fast as it is generated

17. K pH<2 H2O Stomach Lumen ~ Carbonic Anhydrase H Cl H2CO3 CO2 + H2O HCO3 pH>7.4 CO2 Cl Blood

18. Gastric glands – parietal cells • The secretions of the parietal cells keep the stomach pH at a level of 1.5 – 2.0. The functions of such a low pH are: • Kills most microorganisms ingested with the food. • Denatures proteins and inactivate the enzymes in the food • Helps break down plant cell walls and CT in meat • Essential for activation of pepsin – protein-digesting enzyme

19. Gastric glands – chief cells • Chief cells found mostly near the base of the gastric gland. • Secrete pepsinogen which is an inactive proenzyme. • The pepsinogen is converted by the acid in the stomach to pepsin • Pepsin is a protein-digesting enzyme works best in pH of 1.5-2.0 (changes in 3D structure that reveal the active site)

20. Pyloric glands Two types of cells: • Mucos secreting cells • Enteroendocrine cells – hormone secreting cells • G cells secrete gastrin • stimulates secretion of parietal and chief cells • Stimulates contraction of the gastric wall to promote mixture • D cells secrete somatostatin • Inhibits secretion of gastrin

21. Digestion in the Stomach • The stomach: • Holds ingested food • Breaks this food both physically and chemically • Delivers chyme to the small intestine • Enzymatically digests proteins with pepsin • Secretes intrinsic factor required for absorption of vitamin B12

22. Digestion and absorption in the stomach • Preliminary digestion of proteins by pepsin (not completed). Pepsin breaks down complex proteins into smaller peptides • Permits digestion of carbohydrates - until pH fall below 4.5 the salivary amylase continue its function. • Very little absorption of nutrients: • The epithelial cells are covered with mucus • Epithelial cells lack transport mechanisms • Gastric lining is relatively impermeable to water • Most carbohydrates, lipids and proteins are too big (not completely broken down) • Some drugs, however, are absorbed (ethyl alcohol, aspirin)

23. Regulation of Gastric Secretion • Cephalic (reflex) phase: few minutes prior to food entry • Gastric phase: after food enters the stomach • Intestinal phase: brief stimulatory effect as partially digested food enters the duodenum, followed by inhibitory effects (enterogastric reflex and enterogastrones)

24. Cephalic phase ("wake up call"): prepares mouth and stomach for food • Centers in the cerebral cortex, hypothalamus and brain stem are activated by smell, sight, taste or thought of food • Glossopharengeal (IX) and facial (VII) nerves stimulate salivary glands • Vagus (X) stimulate gastric glands (both directly and via ENS) • Parietal cells secrete small amounts of acid and G cells secrete gastrin: • Promote function of parietal and chief cells and gastric motility

25. Figure 24.15a

26. Gastric phase ("full steam ahead"): When a meal enters the stomach • Expansion excites stretch receptors and pH changes (increased by bolus arriving to the stomach) activate chemoreceptors in the mucosa (ENS sensory receptors). • stimulates both G cells (gastrin) and parietal cells (HCl) • Gastrin from the G cells stimulates the parietal and chief cells (pepsinogen). • Enteric nervous system and gastrin promote smooth muscle contractions. • The net result is : secretory and motor functions of the stomach are fully turned on

27. Figure 24.15b

28. Intestinal phase ("step on the brakes"): starts when food enters the small intestine • small intestine slows down gastric emptying, to allow it time to neutralize the acid and efficiently absorb incoming nutrients. • During this phase the small intestine sends inhibitory signals to the stomach to slow secretion and motility. • Distension of the stomach is reduced as a result of the movement of chyme into the small intestine. • At the same time, expansion of the duodenum stimulate stretch receptors that result in the inhibition of parasympathetic activity - enterogastric reflex.

29. Arrival of chyme to duodenum stimulates hormones secretion (hormonal control): • Lipids stimulate cholecystokinin (CCK) and gastric inhibitory peptide (GIP) – both inhibit gastric function • A drop in pH below 4.5 stimulate secretion of secretin – inhibits parietal and chief cells. At the same time it stimulates the pancreas

30. Small Intestine: Microscopic Anatomy • Structural modifications of the small intestine wall increase surface area • Plicae circulares: deep circular folds of the mucosa and submucosa • Villi – fingerlike extensions of the mucosa • Microvilli – tiny projections of absorptive mucosal cells’ plasma membranes (brush border)

31. Intestinal Juice • Secreted in response to distension or irritation of the mucosa • Slightly alkaline and isotonic with blood plasma • Largely water, enzyme-poor, but contains mucus • Facilitates transport and absorption of nutrients

32. Liver • The largest gland in the body • The falciform ligament: • Separates the right and left lobes anteriorly • Suspends the liver from the diaphragm and anterior abdominal wall • The lesser omentum anchors the liver to the stomach • The gallbladder rests in a recess on the inferior surface of the right lobe

33. The hepatic portal system begins in the capillaries of the digestive organs and ends in the portal vein. Consequently, portal blood contains substances absorbed by the stomach and intestines. Portal blood is passed through the hepatic lobules where nutrients and toxins are absorbed, excreted or converted.

34. Liver: Microscopic Anatomy • Liver lobules • Hexagonal structural and functional units • Filter and process nutrient-rich blood • Composed of plates of hepatocytes (liver cells) • Longitudinal central vein

35. Liver: Microscopic Anatomy • Hepatocytes’ functions include: • Production of bile • Processing bloodborne nutrients • Storage of fat-soluble vitamins • Detoxification • Secreted bile flows between hepatocytes toward the bile ducts in the portal triads

36. Liver physiology • Removing and excreting body wastes, hormones, drugs and other foreign substances Enzymes in the liver alter some toxins so they can be more easily excreted in urine. • Synthesizing plasma proteins, including those necessary for blood clotting Most of the 12 clotting factors are produced by the liver. Other plasma proteins produced by the liver include albumin, fibrinogen and certain globulins which transport substances such as cholesterol and iron. • Producing bile to aid in digestion Bile salts aid in fat digestion and absorption. Bile is continuously secreted by the liver and stored in the gallbladder until a meal, when bile enters the beginning of the small intestine.

37. Liver physiology • Excretion of bilirubin Bilirubin is one of the few waste products excreted in bile. Macrophages in the liver remove worn out red blood cells from the blood. Bilirubin results from the breakdown of the hemoglobin is excreted into bile by hepatocytes. • Storing certain vitamins, minerals, and sugars The liver stores enough glucose in the form of glycogen to provide about a day's worth of energy. • The liver also stores fats, iron, copper, and many vitamins including vitamins A, D, K, and B12. • Processing nutrients absorbed from digestive tract • The liver converts glucose into glycogen, its storage form. • The fatty acids produced by the digestion of lipids are used to synthesize cholesterol and other substances.

38. Liver: bile secretion • Bile leaves the liver via: • Bile ducts, which fuse into the common hepatic duct • The common hepatic duct, which fuses with the cystic duct • These two ducts form the bile duct

39. Composition of Bile • A yellow-green, alkaline solution containing bile salts, bile pigments, cholesterol, neutral fats, phospholipids, and electrolytes • Bile salts emulsify fat to facilitate fat and cholesterol absorption • The main bile pigment is bilirubin, a waste product of heme

40. The Gallbladder • Thin-walled, green muscular sac on the ventral surface of the liver • Stores and concentrates bile by absorbing its water and ions • Releases bile via the cystic duct, which flows into the bile duct

41. Regulation of Bile Release • Acidic, fatty chyme causes the duodenum to release Cholecystokinin (CCK) and secretin into the bloodstream • CCK causes: • The gallbladder to contract • The hepatopancreatic sphincter to relax • secretin transported in blood stimulate the liver to produce bile • Vagal stimulation causes weak contractions of the gallbladder • As a result, bile enters the duodenum

42. Pancreas • Location • Lies deep to the greater curvature of the stomach • The head is encircled by the duodenum and the tail is close to the spleen • Exocrine function • Secretes pancreatic juice which breaks down all food • The pancreas also has an endocrine function – release of insulin and glucagon

43. Composition and Function of Pancreatic Juice • Water solution of enzymes and electrolytes (primarily HCO3–) • Neutralizes acid chyme • Provides optimal environment for pancreatic enzymes • Enzymes • Amylase, lipases, nucleases are secreted in active form but require ions or bile for optimal activity • Proteases secreted in inactive form • Protease activation in duodenum • The pancreas starts to synthesize enzymes before food even arrive to the stomach

44. Control of Pancreatic Function

46. Large intestine histology • The ET of the mucosa contain mainly absorptive and goblet cells. • both cell types are found in long, straight intestinal glands – crypts of Lieberkuhn – that extend the full thickness of the mucosa. • Colon mucosa is simple columnar epithelium except in the anal canal • Anal canal mucosa is stratified squamous epithelium

47. Functions of the Large Intestine • Digestion of enteric bacteria • Reabsorption Vitamins, water, and electrolytes • Propulsion of fecal material toward the anus • Though essential for comfort, the colon is not essential for life

48. Bacterial Flora • The bacterial flora of the large intestine consist of: • Bacteria surviving the small intestine that enter the cecum • Those entering via the anus • These bacteria: • Colonize the colon • Ferment indigestible carbohydrates • Synthesize B complex vitamins and vitamin K

49. Valves and Sphincters of the Rectum and Anus • The anus has two sphincters: • Internal anal sphincter composed of smooth muscle • External anal sphincter composed of skeletal muscle • These sphincters are closed except during defecation

50. Defecation • Distension of rectal walls caused by feces: • Stimulates contraction of the rectal walls • Relaxes the internal anal sphincter • Voluntary signals stimulate relaxation of the external anal sphincter and defecation occurs